A catadioptric zoom relay telescope with a fixed entrance pupil diameter, whose refractive elements are all made from the same type of infrared-transmissive material, and which is capable of achieving a continuously variable focal length over an infrared wavelength band from B to 12 microns while maintaining substantially diffraction-limited imagery on a stationary focal plane throughout a changing focal ratio from less than f/1.5 to more than f/6.0, was disclosed in co-pending U.S. patent application Ser. No. 545,211 filed on June 28, 1990, which is incorporated herein by reference.
The design form for the catadioptric zoom relay telescope disclosed in the aforementioned patent application Ser. No. 545,211 specifies that the primary mirror and all the other optical components of the telescope have spherical surfaces. Asphericity is sometimes introduced into an optical system as a design expedient to improve performance with respect to monochromatic aberrations (particularly spherical aberration), and/or to obtain a reduction in the number of optical components needed to achieve optical performance equivalent to that of a system having all spherical surfaces.